1. Field of the Invention
The present invention relates to a method for manufacturing positive and negative electrode plates primarily for use in a nonaqueous electrolyte battery.
2. Description of Related Art
With the widespread use of cordless or portable audio-visual devices, personal computers, and the like, the demand for batteries having a smaller size, a lighter weight, and increased energy density as power supply for these devices is increasing. For such purpose, nonaqueous electrolyte batteries such as lithium rechargeable batteries are particularly suitable because of their high energy density.
Positive electrode plates of nonaqueous electrolyte batteries are generally manufactured through the steps illustrated in FIGS. 2A through 2E. First, a powdered positive electrode mixture 1 obtained by adding a conductive material to a positive electrode active material is put into a mixing vessel 2, and a binding additive 3 produced by mixing a binder into a solvent is poured into this mixing vessel 2, as shown in FIG. 2A. Then, as shown in FIG. 2B, the powdered positive electrode mixture 1 and the binding additive 3 in the mixing vessel 2 are thoroughly stirred and kneaded by a stirrer 4 to produce a paste-form positive electrode mixture 7.
As shown in FIG. 2C, the paste-form positive electrode mixture 7 is applied under pressure to both sides of a positive electrode collector 8 (composed, for example, of a sheet having a fibrous mesh structure or a band-like metal foil), forming a positive electrode mixture layer 10 that is supported on the positive electrode collector 8. The mixture layer 10 is then placed in a drying chamber 9 as shown in FIG. 2D, where it is heated for 2 hours at a temperature of 150xc2x0 C., for instance, thereby being dried.
During the heating, the solvent in the binding additive 3 evaporates away, leaving the active material and the conductive material bonded to the positive electrode collector 8 by the binder. Finally, the positive electrode mixture layer 10 is press-molded to a specific thickness in the calendering step shown in FIG. 2E. After this, the positive electrode collector 8 is punched out or cut to the required size to create the desired positive electrode plate.
For the positive electrode active material of a nonaqueous electrolyte battery, LiCoO2 with an average particle size of just a few microns, is generally used, whereas the conductive material is acetylene black or natural graphite. For the binder, it has been proposed in the past that a powder of polyethylene (hereinafter referred to as PE) resin be used. (See Japanese Laid-Open Patent Applications 4-249861, 7-161348, and 8-273669, respectively.)
PE is known, however, as one of the most difficult polymer materials to bond, because there is no solvent in which it has good solubility. When PE is used as a binder, a problem is that the poor adhesive strength of PE leads to cracking of the positive electrode plates formed by calendering. In prior art this has been dealt with by raising the adhesive strength of PE by heating the positive electrode collector 8 at a high temperature in the drying step following the formation of the positive electrode mixture layer 10. Unfortunately, PE, which has a low melting point, is completely melted by this high temperature heating, and the PE in this completely molten state adheres around the positive electrode active material. Consequently the chemical reaction between the LiCoO2 serving as the positive electrode active material and the carbon material serving as the negative electrode material is hampered by the PE adhering around the positive electrode active material, resulting in diminished battery performance.
One possible way to raise the adhesive strength of PE without heating to a high temperature is to increase the PE content in the paste-form positive electrode mixture 7. In specific terms, PE with a particle size of at least 50 xcexcm has previously been used, the PE content in the paste-form positive electrode mixture 7 being set higher to about 15 to 25 wt %. In this case, not only is it wasteful to increase the amount of binder, which does not contribute anything to battery function, but the mixing ratio of the positive electrode active material per unit of volume in the paste-form positive electrode mixture 7 is also reduced in proportion to the increase in the amount of binder, which leads to a decrease in the discharge capacity per unit of volume of the battery.
Polytetrafluoroethylene (hereinafter referred to as PTFE), polyvinylidene fluoride (hereinafter referred to as PVDF), and other fluorine-based resin powders have also been used in recent years as binders. This is because PTFE and PVDF have higher adhesive strength than PE, and when melted by high temperature heating in the drying step they enter a porous, fibrous state, so an advantage is that the chemical reaction between the positive electrode active material and the negative electrode active material is hardly hampered at all.
Nevertheless, the above-mentioned PTFE and PVDF used as binders undergo a chemical reaction with the electrolytic solution during the use of the battery, resulting in defluorination, and this compromises the adhesive strength of the positive electrode mixture layer 10 to the positive electrode collector 8. Particularly in rechargeable batteries, because the electrode plates are repeatedly subjected to volumetric expansion and contraction during charging and discharging, the above-mentioned decrease in adhesive strength can cause the active material particles or the conductive material to fall out of the positive electrode collector 8, or the particles to be widely separated, which leads to a drop in conductivity. This is one of the factors that can shorten the cycling life of a battery, and also poses problems with the shelf life of a battery. Furthermore, in addition to their high raw material cost, PTFE and PVDF take a long time to be kneaded with the powder positive electrode mixture 1, and this drives up the manufacturing cost, resulting in the increase in the price of positive electrode plates of the nonaqueous electrolyte battery.
The present invention has been devised to solve the above-described problems encountered in the past, and it is an object thereof to provide an improved method for manufacturing battery electrode plates, wherein a polyolefin-based resin such as PE, which is a relatively inexpensive material and is stable with respect to electrolyte, is used as the binder, the adhesive strength of the binder being raised without heating it to a high temperature or increasing the amount of resin contained in the binder.
The method for manufacturing battery electrode plates according to the invention includes mixing a solvent into a polyolefin-based resin used as a binder, heating the mixture of the polyolefin-based resin and the solvent at a temperature at which at least part of the polyolefin-based resin will melt, thereby producing a viscous, gelled adhesive solution, kneading a mixture of a conductive material, an active material, and the adhesive solution to produce a paste mixture, coating a collector with the paste mixture to form a mixture layer, heating and drying the collector on which the mixture layer is formed, and press-molding the mixture layer to a specific thickness.
Preferably, the adhesive solution produced by heating to a specific temperature is mixed with the active material and the conductive material after first being rapidly cooled to a temperature between xe2x88x92175xc2x0 C. and 30xc2x0 C.
The temperature in the heating and drying step is set to be over the boiling point of the solvent in the adhesive solution and under the melting point of the polyolefin-based resin.
Most preferably, the polyolefin-based resin is polyethylene, and the mixture of the polyethylene and the solvent is heated to a temperature between 30 to 160xc2x0C.
While novel features of the invention are set forth in the preceding, the invention, both as to organization and content, can be further understood and appreciated, along with other objects and features thereof, from the following detailed description and examples when taken in conjunction with the attached drawings.